High pulse energy laser systems are generally built around laser oscillator-laser amplifier configurations. Typically, such laser systems include a low-power laser oscillator followed by a laser amplifier that increases the pulse energy to the desired energy level. Frequently used high-gain laser amplifier media is for example Nd:YAG media. The Nd:YAG media and other similar medias have a tendency for Amplified Spontaneous Emission (ASE). ASE is produced when a laser amplifier medium is pumped to produce a population inversion. Feedback of the ASE into the laser's optical cavity may produce unintended laser operation if the lasing threshold is reached. Excess ASE and spurious lasing is an unwanted effect in lasers, since it limits the maximum gain that can be achieved in the gain medium. ASE also depletes the stored energy in the medium and therefore reduces the potential amplification factor.
There are a number of methods and devices employed to reduce the stray photons from entering the amplifier medium and depleting the gain.
The most common reason for the photons emitted as fluorescence to re-enter the laser amplifier medium is due to unintended reflections from surfaces of the optical elements in the laser beam path and even from the surfaces of optical mounts. One of the commonly used techniques for prevention of reflections is to tilt the optics at small angles, for example 0.7 or 0.8 degrees so as to deflect the reflections out of the path of the laser beam. Although widely used, this technique places the optical elements in an off-axis arrangement and adversely affects the quality of the laser beam.
Another technique to reduce the stray photons to re-enter the amplifier medium involves use of Faraday isolators. Faraday isolators are employed to selectively block reflected light, although the isolators maintain a common optical axis with the rest of the optical system. However, Faraday isolators are only applicable to polarized laser light. The polarization properties of the reflected light could be different from that of the incident light. The effectiveness of employing Faraday isolators to attenuate laser light reflected from surface of optical elements is limited.
Faraday isolators have relatively low transmission and their use in high power lasers system reduces energy efficiency and could even result in the isolator damage by the absorbed energy.
Both industry and science are in need of high power, pulsed laser systems and the parasitic optical feedback could be a limiting factor in system operation. Thus, improved methods and systems for reducing stray light reflected from different surfaces in the laser beam path are desired.